The present invention relates generally to digital loop carrier systems and, more particularly, the invention relates to an interface for a digital loop carrier telephone system in which an analog loop within the system is allowed to have a floating ground reference.
Digital loop carrier systems fall under the general category of "carrier systems, subscriber loop." Thus, digital loop carriers ("DLCs") transport telephone services for residential and business communications.
Subscriber loop carrier systems apply basic carrier system principles to increase circuit carrying capacity of a single transmission media through multiplexing techniques. The transmission media for subscriber loop carrier systems has historically been copper loop feeder cables, although there is now an increasing use of fiber optic cables.
A digital telecommunications transmission facility may include a central office terminal (or "COT"), remote terminal (or "RT"), and customer premises equipment. The COT may transmit the digital data signals over digital transmission lines to the RT and, thereafter, to the customer premises equipment. Repeaters may be required on the facility between the COT and RT to regenerate the high frequency carrier signal.
The RT is generally connected to a short length of copper feeder cable for distribution of the derived carrier channels. Subscriber loop carrier systems are often referred to as pair gain systems because the telephone company assigns additional subscriber feeder pairs available for service assignment.
Typically, signals are sent from the COT to a RT over transmission lines differentially over two pairs of cables. The Bell telephone system in the United States, for example, has widely utilized digital "D" multiplexing code modulation systems. One pair of cables is provided for each direction of transmission.
Data within the central office may be communicated, via an analog loop, from a central office switch to a central office terminal. The central office terminal receives the signal from the central office switch and multiplexes a signal for transmission on the digital line to the remote terminal. The digital data is then converted to an analog signal and then provided by the remote terminal to the customer premises on another tip-ring conductor pair.
The analog loop within the central office may include a tip-ring pair. The tip may be connected, for example, to the positive side of a 48 volt battery or other substantially direct current power sources. The tip of the central office loop may be interconnected to a fixed ground reference. Thus if the battery, or other power supply employed by the central office, is, for example, at 48 volts, the ring lead would be at an electrical potential of -48 volts with respect to the ground the general fixed ground reference. If the tip is not tied to a fixed ground reference, however, (which may occur, for example, with the Bell 5ESS system) the tip lead may be allowed to "float" with respect to the ground, and is not tied to the fixed central office ground potential.
In such a case, the central office may interpret its own on hook signal as a "forward disconnect" (because of the floating battery) signal, requiring the remote terminal to open the customer loop. However, in some instances, an on hook signal may simply mean that the customer is using a rotary dial telephone or using a flash book signal (briefly pressing down on the telephone cradle button). Thus; use of a floating ground in the central office precludes the associated subscribers' use of a rotary dial telephone or of the flash hook signal.
Recently, there have been additional requirements for the DLC system to transport voice-bank signals when the telephone is in the on-hook state (with the telephone handset in the cradle). Such requirements include the capability to handle caller identification signals, and to pass open-switching-intervals ("OSI") towards the customer equipment. Such open circuits, or OSIs, are an additional network signaling state that may be used to communicate to customer premises equipment. The requirements for OSIs (also described as Loop Current Feed Open Intervals) are described, for example, in Bellcore Technical Requirement Publication TR-NWT-000057, Issue 2, January 1993, Functional Criterial for DLC Systems, Signalling and Supervision Criteria, paragraph 5.3.10, pages 5-25 to 5-26.
Previously, there existed two specific channel units (within a remote terminal) that were used to transport telephone service. The POTS (Plain Old Telephone Service) channel unit handled residential loop-start applications, and the channel unit handled ground-start applications for business use. The central office terminal (COT) POTS channel unit did provide on-hook transmission and operated with floating switch interfaces, but did not meet the OSI criteria.
The COT Special POTS ("SPOTS") channel unit (with the capability of ground start) also provided on hook transmission, but did not operate with a floating switch interface and did not meet the OSI criteria. COT SPOTS channel units that met the OSI requirements often could not pass rotary dial or hook-flash signals from the customer equipment to the floating switch interface without prematurely disconnecting the call set up sequence.
Thus, ability to pass a hook-flash signal on a floating switch interface and allow OSI signals were frequently mutually exclusive options. A hook-flash signal towards the network was often reflected as an open circuit condition by the COT SPOTS channel unit. This, in turn, would signal the COT to terminate the transmission link between the office equipment and the telephone.